This invention relates to protection and control systems for steam turbines and in particular to a control system which establishes predetermined acceptable high and low limits of operating pressure and temperature for the inlet steam to the turbine. The system generates a first alarm signal if the pressure or temperature exceeds the predetermined acceptable range of operation, generates a second alarm signal if the rate of change of temperature or pressure, while outside the predetermined acceptable operating range, exceeds a predetermined acceptable rate of change and generates a third alarm signal if the rate of change of pressure or temperature, while outside the acceptable operating range, exceeds a predetermined maximum rate of change of pressure or temperature.
Steam turbines are designed to operate with a supply of steam produced by a boiler. During certain operating conditions, there is a risk that water may enter the steam turbine along with the steam. Water carryover is undesirable since the resulting mechanical vibrations and thermal shock may result in shortened life time of or immediate permanent damage to the steam turbine. Sudden changes in steam pressure may cause water carryover the prior art systems utilize various ways to attempt to control the steam pressure in order to avoid this condition.
Some prior art devices track pressure variations over extended periods of time which require control devices that are stable and drift-free. Other devices detect a reduction in steam pressure in the boiler at a rate exceeding some predetermined threshold value and then change the setting of a steam control valve in response to the detecting means to attempt to control steam pressure.
Accompanying rapid changes in pressure may be a rate of change of temperature. These changes of temperature create thermal stress such as wheels loosening and cases cracking, create distortion and internal misalignment and vibration.
However, there are reasonable ranges of variation of temperature and pressure which ought not to require monitoring and further there are certain rates of change of pressure and temperature which ought to be allowable without attempts to control the pressure and temperature through opening or closing of valves and the like. It is only when some normally acceptable or maximum predetermined rate of change of temperature or pressure is exceeded that danger may occur and the operator alerted.
Therefore, it would be desirable to have a control system in which, first, an acceptable range of variation of temperature and pressure is established so that any variations of temperature or pressure within that range would not be of any concern or called to the attention of the operator. Secondly, the rage of change of pressure or temperature should be monitored only outside that allowable range of operation and then a second alarm produced if the rate of change of temperature or pressure exceeds the predetermined allowable rate of change of pressure. Finally, there should be a predetermined maximum rate of change of pressure or temperature which, if exceeded, should produce a third alarm indicating that a dangerous condition is existing.
In each of these cases, the turbine would not be prematurely shut down or the operator unduly alarmed for acceptable variations in pressure and temperature. However, the operator would be appropriately alerted to a condition that required monitoring.
Thus, it is an object of the present invention to provide a turbine control system in which an acceptable range of variation of temperature and pressure is established and a first alarm produced only when said acceptable range is exceeded.
It is another object of the present invention to provide a second alarm signal only when said pressure or temperature is operating outside the acceptable range of variation and the rate of change of temperature or pressure exceeds a predetermined acceptable value.
It is still another object of the present invention to provide a third alarm signal only when the pressure and temperature are operating outside the acceptable range and when the rage of change of temperature or pressure exceeds a predetermined maximum rate of change.